We used Xenopus oocytes and 3T3 L1 preadipocytes to analyze the mechanism of participation of ras proteins in proliferation and differentiation processes. In Xenopus oocytes we showed that microinjected p21ras can induce germinal vesicle breakdown (GVBD) in the absence of protein synthesis through a mechanism involving activation of cytosolic MPF (p34cdc2) kinase. The injection of p21ras in the presence of cycloheximide also activated kinase(s) prior to MPF which were responsible for the phosphorylation and subsequent enzymatic activation of the endogenous MAP2 kinase and S6 kinase. These results indicate that, in the absence of protein synthesis, p21ras can activate cascades of phosphorylation leading to GVBD and suggest that this oncoprotein can participate in at least two separate pathways of MPF activation. We also showed that insulation-induced differentiation of 3T3 L1 fibroblasts to adipocytes can be mimicked by expression of transfected ras oncogenes but not tyrosine kinase oncogenes like src and trk. Exposure of 3T3 L1 cells to insulin caused formation of the active ras. GTP complex, and expression of dominant negative ras mutants inhibited insulin-induced differentiation. Activation of MAP kinases occurred early after insulin stimulation of ras expression in the cells. These results indicate that p21ras is mediating insulin signalling in these cells. We propose that the activation of extracellular signal-regulated kinases (ERK) is an early effect of p21ras both in proliferation and differentiation processes.